test: default tox to 3.8

[jnumainville]

fixes #965

Default to test with 3.8

Also adds other environments to run. A tiny bit more maintenance when we add/remove version support but useful for both the gh action and easily running different versions locally

[mattrunyon]

I would change the PR title to test instead of fix

How does this affect users? Will tox install Python for you or should we update the readme to suggest what version of Python to install or suggest something to manage different Python versions?

I guess since it uses the versioned Python executable we could just recommend installing the specific version for testing.

[jnumainville]

tox sets up an independent python installation so it doesn't matter which one is currently being used. What this does now is allow you to easily test with different versions
Currently, if you just run with tox -e py it will use whatever version you are using in your environment.
With this change, if you run tox -e py it will always run 3.8. You can also run specific other versions easily though, such as tox -e py3.9, tox -e py3.10 etc.

[mattrunyon]

Is there something I'm missing for tox to set up my Python version? I just ran tox -e py with this branch and it quit with "could not find python interpreter with spec(s): python3.8"

[jnumainville]

Oh yeah, you do need to have whatever python version you're using already. I can add that note to the tox directions. I should expand on those anyways.
How you have that version is currently dependent on your setup though. Maybe we could standardize that.